Authentication in Servant: Design space exploration

Authentication.hs

--  The idea: generalise authentication to session information which can be
--  extracted from (e.g.) the current request and a suitable monad 'm'. This
--  is currently problematic as no 'm' is exposed through the type class
--  head.
instance (HasServer sublayout)
      =>  HasServer (WithSession st :> sublayout) where
      
  type ServerT (WithSession st :> sublayout) m
    = MonadSessionWith st m => ServerT sublayout m

  route Proxy subserver request respond
    = route (Proxy :: Proxy sublayout) subserver request respond
    
class Monad m => MonadWithSession st m where
  fromSession_ :: Request -> m st
  
--  Example desired use case (equally vapourware):

newtype SessionId
  = SessionId Text
  deriving (Eq, Ord, Show)
  
deriving instance FromJSON SessionId
deriving instance ToJSON SessionId

type API
  = WithSession SessionId :> "/session" :> Get '[JSON] SessionId
  
getSessionId :: MonadSessionWith SessionId m => ServerT API m
getSessionId
  = fromSession
  
--  Here, 'fromSession' calls 'fromSession_' with the current request,
--  which it somehow gets access to. Perhaps the reflection library might
--  be of some use here? The idea being that the 'route' function in the
--  'WithSession' instance sets up some temporary type class instance
--  which exposes the desired state in a parameterless manner?